enhance: Add comprehensive .gitignore for deb-mock project

- Add mock-specific build artifacts (chroot/, mock-*, mockroot/)
- Include package build files (*.deb, *.changes, *.buildinfo)
- Add development tools (.coverage, .pytest_cache, .tox)
- Include system files (.DS_Store, Thumbs.db, ._*)
- Add temporary and backup files (*.tmp, *.bak, *.backup)
- Include local configuration overrides (config.local.yaml, .env.local)
- Add test artifacts and documentation builds
- Comprehensive coverage for Python build system project

This ensures build artifacts, chroot environments, and development
tools are properly ignored in version control.

docs/Release-Notes-Configs-39.3.md

@@ -0,0 +1,27 @@
+---
+layout: default
+title: Release Notes - Mock Core Configs 39.3
+---
+
+## [Release 39.3](https://rpm-software-management.github.io/mock/Release-Notes-Configs-39.3) - 2023-12-01
+
+### Mock Core Configs changes
+
+- Per the approved [Fedora 40 change](https://fedoraproject.org/wiki/Changes/BuildWithDNF5),
+  [we switched][PR#1256] the default `package_manager` configuration for Fedora 40
+  (Rawhide at that point in time) to `dnf5`.  DNF5 is [the future replacement for
+  DNF4](https://fedoraproject.org/wiki/Changes/ReplaceDnfWithDnf5), aiming to be
+  much faster than its predecessor.  Hence, the effect of this change is a
+  significantly faster buildroot preparation.
+- The default `fedora-eln-*` bootstrap image `quay.io/fedoraci/fedora:eln`
+  [has been fixed](https://github.com/fedora-eln/eln/issues/166) to provide
+  the `dnf builddep` command.  It means it is now "ready for bootstrap" right
+  after the image download (no additional packages need to be installed inside)
+  which makes the buildroot preparation
+  [much faster](https://rpm-software-management.github.io/mock/Feature-container-for-bootstrap).
+- The OpenMandriva chroots provide `python-dnf` and `python-dnf-plugins-core`
+  packages, not `python3-dnf` and `python3-dnf-plugins-core`.  That's why we
+  [had to fix][issue#1251] the `dnf_install_command` config option appropriately.
+
+[PR#1256]: https://github.com/rpm-software-management/mock/pull/1256
+[issue#1251]: https://github.com/rpm-software-management/mock/issues/1251